The Dark Star hypothesis sees the light of day

Physics 16, 127

Recent data from the JWST space observatory has identified several objects consistent with dark matter-powered stars.

NASA; ESA; CSA; B. Robertson; B.Johnson; S. Tacchella; M.Rieke; D. Eisenstein

This infrared image is from the JWST Advanced Deep Extragalactic Survey, or JADES, program. A new analysis of JADES data has identified three objects that could meet the dark star criteria: giant gas clouds heated by dark matter annihilations.This infrared image is from the JWST Advanced Deep Extragalactic Survey, or JADES, program. A new analysis of JADES data has identified three objects that could meet the dark star criteria: gigantic clouds of gas heated by the annihilation of dark matter… Show more

The first stars were probably very different from the stars that light up our nights. The common assumption is that the first generation of stars were large, up to a hundred times more massive than our Sun, and that they glowed hot and bright due to nuclear reactions in their cores. Another possibility is that these stars were powered by dark matter. Recent results from the JWST space observatory have identified three objects from the early Universe that could potentially be supermassive dark stars weighing a million times the mass of our Sun [1]. Experts say more evidence is needed to confirm the sightings.

The ingredients for star formation have evolved considerably throughout the history of the Universe. The first stars formed about 200 million years after the big bang from clouds of gas made up almost entirely of hydrogen and helium: the heavier elements only arrived later, after several cycles of star formation. The lack of heavy elements impacted how these clouds cooled and collapsed under gravity. Models suggest that the gas in the clouds eventually became dense and hot enough to trigger nuclear fusion, just as it does in present-day stars. However, astrophysicists predict that these first stars, called Population III stars, were extremely massive and short-lived.

Astronomers have yet to directly observe Population III stars, so it remains an open question whether the first stars really formed this way. In 2008, Katherine Freese, now at the University of Texas at Austin, and her colleagues considered another possibility: that the first stars were powered by dark matter [2]. Initially, the researchers weren’t sure these objects would emit enough light to be visible, so they called them dark stars. Next, they calculated that a single dark star could be as bright as a galaxy. “The name dark star turned out to be a misnomer,” says Freese. “But we liked it because it’s also the title of songs by the Grateful Dead and Crosby, Stills and Nash.”

The model by Freese and his colleagues assumes that dark stars form in clouds made up mostly of hydrogen, with dark matter contributing only 0.1 percent of the mass. When two of these dark matter particles collide, they can annihilate each other, releasing photons, electrons and other particles. Most of these byproducts remain in the cloud, depositing heat in its gas and causing it to glow, just like a normal star. “These dark stars are really atomic stars with the ‘power of darkness,'” says Freese.

“Dark stars are a theoretically intriguing idea,” says Julian Muñoz, a cosmologist at the University of Texas at Austin who was not involved in the current work. “Self-annihilation is a fairly generic signature of many dark matter models,” he says. And they’re a central feature in a popular model, in which dark matter is made up of massive weakly interacting particles, or WIMPs.

WIMPs self-destruct at a rate proportional to their density. According to the model by Freese and his colleagues, WIMP’s self-annihilation would have been a significant source of energy in gas clouds of the early Universe with masses roughly equivalent to our Sun. The resulting dark stars would have had surface temperatures of about 10,000 K, enough to make them shine brightly but not enough to prevent further matter from accumulating on them. In their models, the researchers found that dark stars can grow to have the mass of a million suns and can emit the light of a billion suns. These supermassive dark stars are what Freese and his team now say they have found in the JWST data.

JWST’s powerful lens has so far revealed more than 700 objects that appear to date back to the earliest epoch of star formation. This number is uncomfortably large, Freese says, because standard cosmological models don’t predict such a bright early Universe. At present, it is unclear what these objects are, as the data is not precise enough to make any identification. However, the JWST team performed follow-up spectroscopic measurements on a small sample of these objects [3]. Data from five objects in this sample has been made public, and based on the analysis of Freese and his colleagues, three of those five may be dark stars.

“This is a really exciting development,” says particle physicist Pearl Sandick of the University of Utah. “Many of us have been eagerly awaiting this type of analysis, which could only be done once adequate spectral information is available from JWST.” For an object to be a dark star, it should have a thermal (black body) spectrum, which all three dark star candidates appear to have. But the spectra are also consistent with those of galaxies filled with normal stars. “These objects can be explained using standard galaxy models,” says astronomer Marcia Rieke of the University of Arizona, who was part of the team that collected the spectral data. She admits, however, that the quality of the spectra still isn’t good enough to rule out dark stars and other exotic explanations.

One way to unambiguously identify these objects would be to take more data and look for spectral characteristics. If the objects are dark stars, their spectra should have a helium absorption line at 164 nm. Find that line and we’ll have the “smoking gun” of a dark star, Freese says. It may not be practical to look for this spectral feature in the current crop of objects, but JWST will see more candidates in the future. One of them is almost guaranteed to be magnified by gravitational lensing, Freese says. “Once you zoom in on one of those guys, you’ll get better information…that’s what it will take to confirm the existence of a dark star.”

This evidence will likely be needed for dark stars to catch on with astronomers like Rebecca Bowler of the University of Manchester in the UK. “I would say most astronomers would support the standard picture that the first stars were Population III stars,” she says. Bowler says recent JWST observations have yielded some exciting hints about Population III stars, which she says makes them the strongest candidate for early stars. “I would caution that we don’t know what dark matter is,” she adds. “And so these models are already by their nature highly uncertain.”

Freese agrees that Population III’s stars are a better bet, but thinks the two models aren’t mutually exclusive. “If the dark matter is a WIMP, you’ll get the dark stars first, whether you like it or not,” he says. “Then sure, along the way, you’ll get the regular Population III stars.”

Michael Schirber

Michael Schirber is Corresponding Editor forPhysics magazine based in Lyon, France.

References

  1. C. Ilie et al.Supermassive Dark Star candidates seen by JWST, Proc. Natl. Acad. Ski. USA 120 (2023).
  2. D. Spolyar et al.Dark matter and the first stars: a new stage in stellar evolution, Phys. Rev. Lett. 100051101 (2008).
  3. BE Robertson et al.Identification and properties of intense star-forming galaxies at redshifts zz>10, Nat. Astron. 7611 (2023).

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